Radio Frequency Identification (RFID) systems are in wide and growing use today. FIG. 1 (prior art) is a schematic diagram depicting an example RFID system including an interrogator and a transponder. In some RFID systems the “identifying” information is written into the transponder during manufacture and never changed, making the interrogator merely a reader. In other systems, however, the identifying information in the transponder can be changed and the interrogator used can then be both a reader and a writer. In view of this, the term “interrogator” is used herein to generically mean a reader, a writer, or both. The terms “transponder” and “tag” have become almost synonymous and are used herein as such.
RFID tags are generally classified by whether they are active or passive. A passive-type RFID tag includes transponder circuitry and an antenna, while an active-type RFID tag additionally includes a power source, such as a battery, fuel-cell, or some equivalent. The circuitry in transponders today is usually embodied in a single integrated circuit, hence the term “transponder chip” is often used.
FIG. 1 depicts a passive-type RFID tag. Such tags are used primarily as the examples in this discussion because they are the most commonly used type today and because they will usually benefit more noticeably by use with the present invention. Nonetheless, it should be appreciated that many active-type RFID designs will also benefit by use of the invention.
A passive-type RFID tag extracts energy from an externally provided radio frequency (RF) wave. Typically this RF wave is an interrogation signal being used to excite the antenna to read or write information in the tag. The lack of a built-in energy source tends to make passive RFID tags cheaper to manufacture, longer lasting, and more reliable. This also tends to make them environmentally friendly, because they do not include the environmentally unfriendly substances typically used in power sources. The lack of a built-in energy source, however, also limits the effective operating range of a passive RFID tag with respect to the given energy in an interrogation signal. For example, to increase operating range or if signal propagation between an interrogator and a tag is somehow limited, it follows that the interrogator being used with a passive RFID tag will have to radiate the interrogation signal at a higher power level to accomplish the task at hand.
Unfortunately, simply increasing the power level of an interrogation signal to insure successful interrogator-tag communications is not always possible. For instance, simple inefficiency can exacerbate problems such as battery life and heat dissipation in the interrogator. Of more serious concern, RF energy radiation intended for one system can interfere with other electronic systems and, in extreme cases, can be unsafe for biological systems, e.g., humans, animals, plants, etc. For this reason, most governments limit RF energy radiation levels, and the United States and both Europe are notable in this respect.
With reference again to FIG. 1, the exemplary passive-type RFID tag depicted there includes an antenna, a matching network, a modulator, a rectifier, and a logic sub-circuit. The antenna is a dipole-type, as is frequently used in RFID systems today. The matching network is shown in a dashed outline because it is optional, as discussed below. The circuitry depicted in the matching network, modulator, and rectifier in FIG. 1 is merely representative, and no circuitry is depicted in the logic sub-circuit because such can vary considerably and is not particularly germane to this discussion. The antenna and the matching network usually must be implemented in discrete components, but the modulator, rectifier, and logic sub-circuit are usually implemented today as a single integrated circuit “transponder chip.”
Temporarily ignoring the matching network, the antenna is connected to the “front end” of the transponder chip and the electromagnetic field of an interrogation signal that impinges on the antenna must produce an output signal having a voltage above a given threshold before the transponder chip can rectify it.
Rectifying the received interrogation signal can serve multiple purposes. In a passive RFID tag it provides the power needed to operate the logic sub-circuit, and ultimately also the modulator that permits the transponder to “send” its identifying information back to an interrogator as backscatter radiation. Additionally, rectification demodulates the interrogation signal if it is providing information to the transponder. This is so if the logic sub-circuit is being programmed, either with the identifying information that the transponder will “reply” with when later read, or with any other programming that the logic sub-circuit can accept. Additionally, the RF carrier of an interrogation signal may include some indication to transponders that it works with that it is a valid interrogation signal, e.g., a particular sub-carrier frequency. This permits the transponder to remain silent when energized by other, invalid RF signals.
To maximize the voltage produced by the antenna, and to thus increase the energy provided to the transponder, the impedance between the antenna and the transponder should match at the operating frequency of the particular interrogation signal being used. One known approach to improving this impedance matching is to utilize circuits of either discrete components, e.g., inductor and capacitor networks or distributed elements such as microstrip structures. The inductor and capacitor matching network shown in FIG. 1 is an example. Unfortunately, these approaches are often undesirable because they increase the cost, complexity, and size of the RFID tag and decrease its efficiency.
It would clearly be very desirable be able to produce an RFID tag with an antenna that is directly matched to the RF circuitry of the transponder chip, to make the most of interrogation signals that are received and to increase the possible operating range between the RFID tag and the interrogator it is used with. Unfortunately, this has until now not been possible without accepting other undesirable consequences.
To achieve optimal impedance matching the transponder antenna should be the conjugate of the impedance of the transponder chip. However, due to its use of diodes and capacitors, the rectifier at the front end of the transponder chip has an input impedance that is a complex value in nearly all cases, comprising both a positive real component and a negative imaginary component. The impedance of the antenna therefore should have the same real component and a positive imaginary component. This normally requires an increase in antenna size, in comparison with a resonant dipole, and that especially conflicts with the usual design goals of reducing overall transponder size and cost.